Method and composition for crystallizing G protein-coupled receptors

ABSTRACT

Certain embodiments provide a method for crystallizing a GPCR. The method may employ a fusion protein comprising: a) a first portion of a G-protein coupled receptor (GPCR), where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertion; and c) a second portion of the GPCR, where the second portion comprises the TM6 and TM7 regions of the GPCR.

GOVERNMENT RIGHTS

This invention was made with Government support under contracts NS028471& GM075811 awarded by the National Institutes of Health. The Governmenthas certain rights in this invention.

BACKGROUND

G protein-coupled receptor (GPCR) signaling plays a vital role in anumber of physiological contexts including, but not limited to,metabolism, inflammation, neuronal function, and cardiovascularfunction. For instance, GPCRs include receptors for biogenic amines,e.g., dopamine, epinephrine, histamine, glutamate, acetylcholine, andserotonin; for purines such as ADP and ATP; for the vitamin niacin; forlipid mediators of inflammation such as prostaglandins, lipoxins,platelet activating factor, and leukotrienes; for peptide hormones suchas calcitonin, follicle stimulating hormone, gonadotropin releasinghormone, ghrelin, motilin, neurokinin, and oxytocin; for non-hormonepeptides such as beta-endorphin, dynorphin A, Leu-enkephalin, andMet-enkephalin; for the non-peptide hormone melatonin; for polypeptidessuch as C5a anaphylatoxin and chemokines; for proteases such asthrombin, trypsin, and factor Xa; and for sensory signal mediators,e.g., retinal photopigments and olfactory stimulatory molecules.

GPCRs are of immense interest for drug development.

SUMMARY OF THE INVENTION

A fusion protein is provided. In certain embodiments, the fusion proteincomprises: a) a first portion of a G-protein coupled receptor (GPCR),where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regionsof the GPCR; b) a stable, folded protein insertion, e.g., the amino acidsequence of lysozyme; and c) a second portion of the GPCR, where thesecond portion comprises the TM6 and TM7 regions of the GPCR. Thepolypeptide may be employed in crystallization methods, for example.

In certain embodiments, the stable, folded protein insertion is apolypeptide than can fold autonomously in a variety of cellularexpression hosts, and is resistant to chemical and thermal denaturation.In particular embodiments, the stable folded protein insertion may be aprotein that is known to be highly crystallizable, in a variety of spacegroups and crystal packing arrangements. In certain cases, the stable,folded protein insertion may also shield the fusion protein fromproteolysis between the TM5 and TM6 domains, and may itself be proteaseresistant. Lysozyme is one such polypeptide, however many others areknown.

Also provided is a nucleic acid encoding the above described fusionprotein, and a cell comprising the same. The fusion protein may bedisposed on the plasma membrane of the cell.

Also provided are crystals comprising the above described fusionprotein, folded into an active form.

The above-described cell may be employed in a method comprising:culturing the cell to produce the fusion protein; and isolating saidfusion protein from the cell. The method may further comprisecrystallizing the fusion protein to make crystals which, in certainembodiments, may involve combining the fusion protein with lipid priorto crystallization. In certain embodiments, the fusion protein iscrystallized using a bicelle crystallization method or a lipidic cubicphase crystallization method. The method may further comprise obtainingatomic coordinates of the fusion protein from the crystal.

Also provided is a method of determining a crystal structure. Thismethod may comprise receiving an above described fusion protein,crystallizing the fusion protein to produce a crystal; and obtainingatomic coordinates of the fusion protein from said crystals. In otherembodiments, the method may comprise forwarding a fusion protein to aremote location where the protein may be crystallized and analyzed, andreceiving the atomic coordinates of the fusion protein.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a schematic illustration of a GPCR, showing the canonicaltransmembrane regions (TM1, TM2, TM3, TM4, TM5, TM6, and TM7),intracellular regions (IC1, IC2, and IC3), and extracellular regions(EC1, EC2, and EC3).

FIG. 2 is a schematic illustration of a subject fusion protein, showinga stable, folded protein insertion between the TM5 and TM6 regions of aGPCR.

FIG. 3 Design and optimization of the β₂AR-T4L (β₂-adrenergic receptorT4 lysozyme) fusion protein. A. The sequence of the region of the β₂AR(β₂-adrenergic receptor) targeted for insertion of a crystallizabledomain is shown, and the positions of the junctions between the receptorand T4L (T4 lysozome; in red) for various constructs are indicated. Thesequences that were initially replaced or removed are faded. Red linesare shown after every tenth residue. B. Immunofluorescence images ofHEK293 cells expressing selected fusion constructs. Panels on the leftshows M1 anti-FLAG signal corresponding to antibody bound to theN-terminus of the receptor. Panels on the right show the same signalmerged with blue emission from DAPI (nuclear staining for all cells).Plasma membrane staining is observed in the positive control, D3 and D1,while C3 and D5 are retained in the endoplasmic reticulum.

FIG. 4 Functional characterization of β₂AR-T4L. A. Affinity competitioncurves for adrenergic ligands binding to β₂AR-T4L and wild-type β₂AR.Binding experiments on membranes isolated from Sf9 insect cellsexpressing the receptors were performed as described below. B. β₂AR-T4Lis still able to undergo ligand-induced conformational changes. Bimanefluorescence spectra (excitation at 350 nm) of detergent-solubilizedβ₂AR-T4L and wild-type β₂AR truncated at 365, labeled under conditionsthat selectively modify Cys265^(6.27), were measured after incubatingunliganded receptor with compounds for 15 min at room temperature. Thecartoon illustrates that the observed changes in fluorescence can beinterpreted as a movement of the bimane probe from a more buried,hydrophobic environment to a more polar, solvent-exposed position.

FIG. 5. A. Side-by-side comparison of the crystal structures of theβ₂AR-T4L fusion protein and the complex between β₂AR365 and a Fabfragment. The receptor component of the fusion protein is shown as ablue cartoon (with modeled carazolol as red spheres), while the receptorbound to Fab5 is in yellow. B. Differences in the environmentsurrounding Phe264^(6.26) (shown as spheres) for the two proteins. C.The analogous interactions to the “ionic lock” between the E(D)RY motifand Glu247^(6.30) seen in rhodopsin (right panel, purple) are broken inboth structures of the β₂AR (left panel, colored blue and yellow asabove). Pymol was used for the preparation of all figures.

FIG. 6. Schematic representation of the interactions between β₂AR-T4Land carazolol at the ligand binding pocket. Residues shown have at leastone atom within 4 Å of the ligand in the 2.4 Å resolution crystalstructure.

FIG. 7. The ligand binding pocket of β₂AR-T4L with carazolol bound. A.Residues within 4 Å of the ligand are shown as sticks, with theexception of A200, N293, F289, and Y308. Residues that form polarcontacts with the ligand (distance cutoff 3.5 Å) are in green, otherresidues are gray (in all panels, oxygens are colored red and nitrogensare blue). B. Same as panel A, except that the ligand is oriented withits amine facing out of the page. W109 is not shown. C. Packinginteractions between carazolol and all residues within 5 Å of theligand. View is from the extracellular side of the membrane. Carazololis shown as yellow spheres, receptor residues are shown as sticks withinvan der Waals dot surfaces. Val114^(3.33), Phe193^(5.32), andPhe290^(6.52) are colored red, all other residues are gray. D. Model of(−)-isoproterenol (magenta sticks) in the ligand binding pocket observedin the crystal structure. A model of the agonist with optimal bondlengths and angles was obtained from the PRODRG server, and the dihedralangles were adjusted to the values observed in the homologous atoms ofbound carazolol (16-22 in FIG. 6). The one remaining unaccounteddihedral in (−)-isoproterenol was adjusted in order to place thecatechol ring in the same plane as the C₁₆-C₁₅—O₁₄ plane in carazolol.Residues known to specifically interact with agonists are shown as greensticks.

FIG. 8. Packing interactions in the β₂AR that are likely to be modulatedduring the activation process. A. On the left, residues previouslydemonstrated to be CAMs or UCMs are shown as van der Waals spheresmapped onto a backbone cartoon of the β₂AR-T4L structure. On the right,residues that are found within 4 Å of the CAMs Leu124^(3.43) andLeu272^(6.34) are shown as yellow spheres or dot surfaces. A verticalcross-section through the structure illustrates that these surroundingresidues connect the CAMs on helices III and VI with the UCMs on helixVII through packing interactions. B. In both β₂AR-T4L (blue) andrhodopsin (purple), a network of ordered water molecules is found at theinterface between the transmembrane helices at their cytoplasmic ends.C. Network of hydrogen bonding interactions between water molecules andβ₂AR-T4L residues (sidechains as blue sticks), notably the UCMs on helixVII (orange cartoon).

FIG. 9A-9M shows the amino acid and nucleotide sequences of exemplarylysozyme fusion proteins.

FIG. 10. Affinity curves for adrenergic ligands binding to β₂AR-T4L andwild-type β₂AR. Saturation curves for the antagonist [³H]DHA is shown atleft, next to competition binding curves for the natural ligand(−)-Epinephrine and the high-affinity synthetic agonist Formoterol.Binding experiments on membranes isolated from Sf9 insect cellsexpressing the receptors were performed as described above.

FIG. 11. Comparison of the proteolytic stability between the wild-typeβ₂AR and β₂AR-T4L in a limited trypsin proteolysis assay. TPCK-trypsinwas added to carazolol-bound, purified, dodecylmaltoside-solubilizedreceptor at a 1:1000 ratio (wt:wt), and samples were analyzed bySDS-PAGE. Intact β₂AR-T4L (56.7 kD) and FLAG-tagged wild-type β₂AR (47.4kD) migrate similarly as ˜55 kD bands. Markers are Biorad low-rangeSDS-PAGE protein standards.

FIG. 12. Stability comparison of unliganded β₂AR365 and β₂AR-T4L. Fordodecylmaltoside-solubilized receptor preparations, maintenance of theability to specifically bind [³H]DHA after incubation at 37° C. is takenas a measure of stability.

FIG. 13. Superimposed Cα traces of the receptor component of β₂AR-T4L(in blue) and β₂AR365 (in yellow). Common modeled transmembrane helixregions 41-58, 67-87, 108-137, 147-164, 204-230, 267-291, 312-326,332-339 were used in the superposition by the program Lsqkab (RMSD=0.8Å).

FIG. 14. Carazolol dissociation from β₂AR365.Dodecylmaltoside-solubilized carazolol-bound receptor (at 50 μM) wasdialyzed in a large volume of buffer containing 300 micromolaralprenonol as a competing ligand, and aliquots were removed from thedialysis cassette at different time points. Remaining bound carazololwas measured (in a relative sense) by collecting fluorescence emissionwith excitation at 330 nm and emission from 335 to 400 nm. For eachcarazolol fluorescence measurement, data was normalized for the proteinconcentration in the dialysis cassette (measured with the Bio-RadProtein DC kit). The Y-axis represents carazolol fluorescence emissionIntensity (in cps) at 341 nm. The exponential decay of carazololconcentration in the receptor dialysis cassette was fit using GraphpadPrism software, giving a half-life of 30.4 hrs.

FIG. 15. After aligning the β₁ and β₂AR sequences, positions that havedifferent amino acids between the two receptors were mapped onto thehigh-resolution structure of β₂AR-T4L (shown as red sticks). Thecarazolol ligand is shown as green sticks (with nitrogens in blue andoxygens in red). Highlighted residues Ala85^(2.56), Ala92^(2.63) andTyr308^(7.35) are homologous to amino acids Leu110^(2.56), Thr117^(2.63)and Phe359^(7.35) of the β₁AR, which were shown to be primarilyresponsible for its selectivity over β₂AR for the compound RO363. In theβ₂AR-T4L structure, only Tyr308^(7.35) faces the ligand, whileAla85^(2.56) lies at the interface between helices II and III. Of allthe divergent amino acids, only Tyr308^(7.35) is found within 4 Å of anyatom of carazolol.

FIG. 16 shows exemplary sequences that may be employed in place of thelysozyme sequences of FIG. 9.

DEFINITIONS

Unless defined otherwise herein, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this invention belongs. Singleton, et al.,DICTIONARY OF MICROBIOLOGY AND MOLECULAR BIOLOGY, 2D ED., John Wiley andSons, New York (1994), and Hale & Marham, THE HARPER COLLINS DICTIONARYOF BIOLOGY, Harper Perennial, NY (1991) provide one of skill withgeneral dictionaries of many of the terms used in this disclosure.Although any methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of the presentinvention, the preferred methods and materials are described.

All patents and publications, including all sequences disclosed withinsuch patents and publications, referred to herein are expresslyincorporated by reference.

Numeric ranges are inclusive of the numbers defining the range. Unlessotherwise indicated, nucleic acids are written left to right in 5′ to 3′orientation; amino acid sequences are written left to right in amino tocarboxy orientation, respectively.

The headings provided herein are not limitations of the various aspectsor embodiments of the invention which can be had by reference to thespecification as a whole. Accordingly, the terms defined immediatelybelow are more fully defined by reference to the specification as awhole.

“G-protein coupled receptors”, or “GPCRs” are polypeptides that share acommon structural motif, having seven regions of between 22 to 24hydrophobic amino acids that form seven alpha helices, each of whichspans a membrane. As illustrated in FIG. 1, each span is identified bynumber, i.e., transmembrane-1 (TM1), transmembrane-2 (TM2), etc. Thetransmembrane helices are joined by regions of amino acids betweentransmembrane-2 and transmembrane-3, transmembrane-4 andtransmembrane-5, and transmembrane-6 and transmembrane-7 on theexterior, or “extracellular” side, of the cell membrane, referred to as“extracellular” regions 1, 2 and 3 (EC1, EC2 and EC3), respectively. Thetransmembrane helices are also joined by regions of amino acids betweentransmembrane-1 and transmembrane-2, transmembrane-3 andtransmembrane-4, and transmembrane-5 and transmembrane-6 on theinterior, or “intracellular” side, of the cell membrane, referred to as“intracellular” regions 1, 2 and 3 (IC1, IC2 and IC3), respectively. The“carboxy” (“C”) terminus of the receptor lies in the intracellular spacewithin the cell, and the “amino” (“N”) terminus of the receptor lies inthe extracellular space outside of the cell. GPCR structure andclassification is generally well known in the art, and furtherdiscussion of GPCRs may be found in Probst, DNA Cell Biol. 1992 11:1-20;Marchese et al Genomics 23: 609-618, 1994; and the following books:Jürgen Wess (Ed) Structure-Function Analysis of G Protein-CoupledReceptors published by Wiley-Liss (1st edition; Oct. 15, 1999); Kevin R.Lynch (Ed) Identification and Expression of G Protein-Coupled Receptorspublished by John Wiley & Sons (March 1998) and Tatsuya Haga (Ed), GProtein-Coupled Receptors, published by CRC Press (Sep. 24, 1999); andSteve Watson (Ed) G-Protein Linked Receptor Factsbook, published byAcademic Press (1st edition; 1994). A schematic representation of atypical GPCR is shown in FIG. 1.

The term “naturally-occurring” in reference to a GPCR means a GPCR thatis naturally produced (for example and not limitation, by a mammal or bya human). Such GPCRs are found in nature. The term “non-naturallyoccurring” in reference to a GPCR means a GPCR that is notnaturally-occurring. Wild-type GPCRs that have been made constitutivelyactive through mutation, and variants of naturally-occurring GPCRs,e.g., epitope-tagged GPCR and GPCRs lacking their native N-terminus areexamples of non-naturally occurring GPCRs.

The term “ligand” means a molecule that specifically binds to a GPCR. Aligand may be, for example a polypeptide, a lipid, a small molecule, anantibody. A “native ligand” is a ligand that is an endogenous, naturalligand for a native GPCR. A ligand may be a GPCR “antagonist”,“agonist”, “partial agonist” or “inverse agonist”, or the like.

A “modulator” is a ligand that increases or decreases a GPCRintracellular response when it is in contact with, e.g., binds, to aGPCR that is expressed in a cell. This term includes agonists, includingpartial agonists and inverse agonists, and antagonists.

A “deletion” is defined as a change in either amino acid or nucleotidesequence in which one or more amino acid or nucleotide residues,respectively, are absent as compared to an amino acid sequence ornucleotide sequence of a parental GPCR polypeptide or nucleic acid. Inthe context of a GPCR or a fragment thereof, a deletion can involvedeletion of about 2, about 5, about 10, up to about 20, up to about 30or up to about 50 or more amino acids. A GPCR or a fragment thereof maycontain more than one deletion.

An “insertion” or “addition” is that change in an amino acid ornucleotide sequence which has resulted in the addition of one or moreamino acid or nucleotide residues, respectively, as compared to an aminoacid sequence or nucleotide sequence of a parental GPCR. “Insertion”generally refers to addition to one or more amino acid residues withinan amino acid sequence of a polypeptide, while “addition” can be aninsertion or refer to amino acid residues added at an N- or C-terminus,or both termini. In the context of a GPCR or fragment thereof, aninsertion or addition is usually of about 1, about 3, about 5, about 10,up to about 20, up to about 30 or up to about 50 or more amino acids. AGPCR or fragment thereof may contain more than one insertion. Referenceto particular GPCR or group of GPCRs by name, e.g., reference to theserotonin or histamine receptor, is intended to refer to the wild typereceptor as well as active variants of that receptor that can bind tothe same ligand as the wild type receptor and/or transduce a signal inthe same way as the wild type receptor.

A “substitution” results from the replacement of one or more amino acidsor nucleotides by different amino acids or nucleotides, respectively ascompared to an amino acid sequence or nucleotide sequence of a parentalGPCR or a fragment thereof. It is understood that a GPCR or a fragmentthereof may have conservative amino acid substitutions which havesubstantially no effect on GPCR activity. By conservative substitutionsis intended combinations such as gly, ala; val, ile, leu; asp, glu; asn,gln; ser, thr; lys, arg; and phe, tyr.

The term “biologically active”, with respect to a GPCR, refers to a GPCRhaving a biochemical function (e.g., a binding function, a signaltransduction function, or an ability to change conformation as a resultof ligand binding) of a naturally occurring GPCR.

As used herein, the terms “determining,” “measuring,” “assessing,” and“assaying” are used interchangeably and include both quantitative andqualitative determinations. Reference to an “amount” of a GPCR in thesecontexts is not intended to require quantitative assessment, and may beeither qualitative or quantitative, unless specifically indicatedotherwise.

The terms “polypeptide” and “protein”, used interchangeably herein,refer to a polymeric form of amino acids of any length, which caninclude coded and non-coded amino acids, chemically or biochemicallymodified or derivatized amino acids, and polypeptides having modifiedpeptide backbones.

The term “fusion protein” or grammatical equivalents thereof is meant aprotein composed of a plurality of polypeptide components, that whiletypically unjoined in their native state, are joined by their respectiveamino and carboxyl termini through a peptide linkage to form a singlecontinuous polypeptide. Fusion proteins may be a combination of two,three or even four or more different proteins. The term polypeptideincludes fusion proteins, including, but not limited to, fusion proteinswith a heterologous amino acid sequence, fusions with heterologous andhomologous leader sequences, with or without N-terminal methionineresidues; immunologically tagged proteins; fusion proteins withdetectable fusion partners, e.g., fusion proteins including as a fusionpartner a fluorescent protein, β-galactosidase, luciferase, etc.; andthe like.

The terms “nucleic acid molecule” and “polynucleotide” are usedinterchangeably and refer to a polymeric form of nucleotides of anylength, either deoxyribonucleotides or ribonucleotides, or analogsthereof. Polynucleotides may have any three-dimensional structure, andmay perform any function, known or unknown. Non-limiting examples ofpolynucleotides include a gene, a gene fragment, exons, introns,messenger RNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA,recombinant polynucleotides, branched polynucleotides, plasmids,vectors, isolated DNA of any sequence, control regions, isolated RNA ofany sequence, nucleic acid probes, and primers. The nucleic acidmolecule may be linear or circular.

As used herein the term “isolated,” when used in the context of anisolated compound, refers to a compound of interest that is in anenvironment different from that in which the compound naturally occurs.“Isolated” is meant to include compounds that are within samples thatare substantially enriched for the compound of interest and/or in whichthe compound of interest is partially or substantially purified.

As used herein, the term “substantially pure” refers to a compound thatis removed from its natural environment and is at least 60% free, atleast 75% free, or at least 90% free from other components with which itis naturally associated.

A “coding sequence” or a sequence that “encodes” a selected polypeptide,is a nucleic acid molecule which can be transcribed (in the case of DNA)and translated (in the case of mRNA) into a polypeptide, for example, ina host cell when placed under the control of appropriate regulatorysequences (or “control elements”). The boundaries of the coding sequenceare typically determined by a start codon at the 5′ (amino) terminus anda translation stop codon at the 3′ (carboxy) terminus. A coding sequencecan include, but is not limited to, cDNA from viral, prokaryotic oreukaryotic mRNA, genomic DNA sequences from viral or prokaryotic DNA,and synthetic DNA sequences. A transcription termination sequence may belocated 3′ to the coding sequence. Other “control elements” may also beassociated with a coding sequence. A DNA sequence encoding a polypeptidecan be optimized for expression in a selected cell by using the codonspreferred by the selected cell to represent the DNA copy of the desiredpolypeptide coding sequence.

“Operably linked” refers to an arrangement of elements wherein thecomponents so described are configured so as to perform their usualfunction. In the case of a promoter, a promoter that is operably linkedto a coding sequence will effect the expression of a coding sequence.The promoter or other control elements need not be contiguous with thecoding sequence, so long as they function to direct the expressionthereof. For example, intervening untranslated yet transcribed sequencescan be present between the promoter sequence and the coding sequence andthe promoter sequence can still be considered “operably linked” to thecoding sequence.

By “nucleic acid construct” it is meant a nucleic acid sequence that hasbeen constructed to comprise one or more functional units not foundtogether in nature. Examples include circular, linear, double-stranded,extrachromosomal DNA molecules (plasmids), cosmids (plasmids containingCOS sequences from lambda phage), viral genomes comprising non-nativenucleic acid sequences, and the like.

A “vector” is capable of transferring gene sequences to a host cell.Typically, “vector construct,” “expression vector,” and “gene transfervector,” mean any nucleic acid construct capable of directing theexpression of a gene of interest and which can transfer gene sequencesto host cells, which can be accomplished by genomic integration of allor a portion of the vector, or transient or inheritable maintenance ofthe vector as an extrachromosomal element. Thus, the term includescloning, and expression vehicles, as well as integrating vectors.

An “expression cassette” comprises any nucleic acid construct capable ofdirecting the expression of a gene/coding sequence of interest, which isoperably linked to a promoter of the expression cassette. Such cassettescan be constructed into a “vector,” “vector construct,” “expressionvector,” or “gene transfer vector,” in order to transfer the expressioncassette into a host cell. Thus, the term includes cloning andexpression vehicles, as well as viral vectors.

A first polynucleotide is “derived from” or “corresponds to” a secondpolynucleotide if it has the same or substantially the same nucleotidesequence as a region of the second polynucleotide, its cDNA, complementsthereof, or if it displays sequence identity as described above.

A first polypeptide is “derived from” or “corresponds to” a secondpolypeptide if it is (i) encoded by a first polynucleotide derived froma second polynucleotide, or (ii) displays sequence identity to thesecond polypeptides as described above.

The term “stable, folded protein insertion” refers to a folded region ofpolypeptide that is inserted between two neighboring domains (e.g., theTM5 and TM6 domains of a GPCR), such that the domains are spacedrelative to each other at a distance that allows them to interact as inthe wild-type protein. The term “stable, folded protein insertion”excludes an amino acid sequence of a fluorescent protein (e.g., GFP, CFPor YFP), and excludes amino acid sequences that are at least 90%identical to the entire IC3 loop of a GPCR. In general, the IC3 loops ofwild type GPCRs do not contain stable, folded protein domains.

The term “active form” or “native state” of a protein is a protein thatis folded in a way so as to be active. A GPCR is in its active form ifit can bind ligand, alter conformation in response to ligand binding,and/or transduce a signal which may or may not be induced by ligandbinding. An active or native protein is not denatured.

The term “stable domain” is a polypeptide domain that, when folded inits active form, is stable, i.e., does not readily become inactive ordenatured.

The term “folds autonomously” indicates a protein that folds into itsactive form in a cell, without biochemical denaturation and renaturationof the protein, and without chaperones.

The term “naturally-occurring” refers to an object that is found innature.

The term “non-naturally-occurring” refers to an object that is not foundin nature.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As noted above, a fusion protein is provided. In certain embodiments,the fusion protein comprises: a) a first portion of a G-protein coupledreceptor (GPCR), where the first portion comprises the TM1, TM2, TM3,TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertionc) a second portion of the GPCR, where the second portion comprises theTM6 and TM7 regions of the GPCR. In particular embodiments, the stable,folded protein insertion spaces the ends of the TM5 region and the TM6region of the GPCR at a distance in the range of 7 Å to 15 Å. Thestable, folded protein insertion may also provide polar surface area forcrystal lattice contacts.

In the following description, the fusion protein is described first,followed by a discussion of the crystallization method in which thefusion protein may be employed.

Fusion Proteins

As noted above, a subject fusion proteins comprises: a) a first portionof a G-protein coupled receptor (GPCR), where the first portioncomprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) astable, folded protein insertion c) a second portion of the GPCR, wherethe second portion comprises the TM6 and TM7 regions of the GPCR. Inparticular embodiments, the stable, folded protein insertion spaces theends of the TM5 region and the TM6 region of the GPCR at a distance inthe range of 7 Å to 15 Å. The stable, folded protein insertion may alsoprovide polar surface for crystal lattice contacts.

In very general terms, such a protein may be made by substituting theIC3 region of the GPCR with a stable, folded protein that holds the tworemaining portions of the GPCR (i.e. the portion that lies N-terminal tothe IC3 region and the portion that lies C-terminal to the IC3 region)together at a distance that is compatible with a functional GPCR interms of pharmacologic and dynamic properties.

GPCRs

Any known GPCR is suitable for use in the subject methods, as long as ithas TM5 and TM6 regions that are identifiable in the sequence of theGPCR. A disclosure of the sequences and phylogenetic relationshipsbetween 277 GPCRs is provided in Joost et al. (Genome Biol. 20023:RESEARCH0063, the entire contents of which is incorporated byreference) and, as such, at least 277 GPCRs are suitable for the subjectmethods. A more recent disclosure of the sequences and phylogeneticrelationships between 367 human and 392 mouse GPCRs is provided inVassilatis et al. (Proc Natl Acad Sci 2003 100:4903-8 andwww.primalinc.com, each of which is hereby incorporated by reference inits entirely) and, as such, at least 367 human and at least 392 mouseGPCRs are suitable for the subject methods. GPCR families are alsodescribed in Fredriksson et al (Mol. Pharmacol. 2003 63, 1256-72).

The methods may be used, by way of exemplification, for purinergicreceptors, vitamin receptors, lipid receptors, peptide hormonereceptors, non-hormone peptide receptors, non-peptide hormone receptors,polypeptide receptors, protease receptors, receptors for sensory signalmediator, and biogenic amine receptors not including β2-adrenergicreceptor. In certain embodiments, said biogenic amine receptor does notinclude an adrenoreceptor. α-type adrenoreceptors (e.g. α_(1A), α_(1B)or α_(1C) adrenoreceptors), and β-type adrenoreceptors (e.g. β₁, β₂, orβ₃ adrenoreceptors) are discussed in Singh et al., J. Cell Phys.189:257-265, 2001.

It is recognized that both native (naturally occurring) and alterednative (non-naturally occurring) GPCRs may be used in the subjectmethods. In certain embodiments, therefore, an altered native GPCR (e.g.a native GPCR that is altered by an amino acid substitution, deletionand/or insertion) such that it binds the same ligand as a correspondingnative GPCR, and/or couples to a G-protein as a result of the binding.In certain cases, a GPCR employed herein may be at least 80% identicalto, e.g., at least 90% identical, at least 85% identical, at least 90%identical, at least 95% identical, or at least 98% identical, to anaturally occurring GPCR.

As such, the following GPCRs (native or altered) find particular use asparental GPCRs in the subject methods: cholinergic receptor, muscarinic3; melanin-concentrating hormone receptor 2; cholinergic receptor,muscarinic 4; niacin receptor; histamine 4 receptor; ghrelin receptor;CXCR3 chemokine receptor; motilin receptor; 5-hydroxytryptamine(serotonin) receptor 2A; 5-hydroxytryptamine (serotonin) receptor 2B;5-hydroxytryptamine (serotonin) receptor 2C; dopamine receptor D3;dopamine receptor D4; dopamine receptor D1; histamine receptor H2;histamine receptor H3; galanin receptor 1; neuropeptide Y receptor Y1;angiotensin II receptor 1; neurotensin receptor 1; melanocortin 4receptor; glucagon-like peptide 1 receptor; adenosine A1 receptor;cannabinoid receptor 1; and melanin-concentrating hormone receptor 1.

In particular embodiments, the GPCR may belong to one of the followingGPCR families: amine, peptide, glycoprotein hormone, opsin, olfactory,prostanoid, nucleotide-like, cannabinoid, platelet activating factor,gonadotropin-releasing hormone, thyrotropin-releasing hormone ormelatonin families, as defined by Lapinsh et al (Classification ofG-protein coupled receptors by alignment-independent extraction ofprinciple chemical properties of primary amino acid sequences. Prot.Sci. 2002 11:795-805) or family B (which includes the PTH and glucagonreceptors) or family C (which includes the GABA and glutamatereceptors).

In the subject methods, the region between the TM5 and TM6 regions of aGPCR (i.e., the IC3 region) is usually identified, and replaced with astable, folded protein insertion to form a fusion protein. The stable,folded protein insertion spaces the TM5 and TM6 regions relative to oneanother. A schematic representation of the prototypical structure of aGPCR is provided in FIG. 1, where these regions, in the context of theentire structure of a GPCR, may be seen. A schematic representation of asubject fusion protein is shown in FIG. 2. In one embodiment, the IC3loop of the GPCR is replaced with a stable, folded protein insertion.

The IC3 region of a GPCR lies in between transmembrane regions TM5 andTM6 and, may be about 12 amino acids (CXCR3 and GPR40) to about 235amino acids (cholinergic receptor, muscarinic 3) in length, for example.The TM5, IC3, and TM6 regions are readily discernable by one of skill inthe art using, for example, a program for identifying transmembraneregions; once transmembrane regions TM5 and TM6 regions are identified,the IC3 region will be apparent. The TM5, IC3, and TM6 regions may alsobe identified using such methods as pairwise or multiple sequencealignment (e.g. using the GAP or BESTFIT of the University ofWisconsin's GCG program, or CLUSTAL alignment programs, Higgins et al.,Gene. 1988 73:237-44), using a target GPCR and, for example, GPCRs ofknown structure.

Suitable programs for identifying transmembrane regions include thosedescribed by Moller et al., (Bioinformatics, 17:646-653, 2001). Aparticularly suitable program is called “TMHMM” Krogh et al., (Journalof Molecular Biology, 305:567-580, 2001). To use these programs via auser interface, a sequence corresponding to a GPCR or a fragment thereofis entered into the user interface and the program run. Such programsare currently available over the world wide web, for example at thewebsite of the Center for Biological Sequence Analysis atcbs.dtu.dk/services/. The output of these programs may be variable interms its format, however they usually indicate transmembrane regions ofa GPCR using amino acid coordinates of a GPCR.

When TM regions of a GPCR polypeptide are determined using TMHMM, theprototypical GPCR profile is usually obtained: an N-terminus that isextracellular, followed by a segment comprising seven TM regions, andfurther followed by a C-terminus that is intracellular. TM numbering forthis prototypical GPCR profile begins with the most N-terminallydisposed TM region (TM1) and concludes with the most C-terminallydisposed TM region (TM7).

Accordingly, in certain embodiments, the amino acid coordinates of theTM5, IC-3, and TM6 regions of a GPCR are identified by a suitable methodsuch as TMHMM.

In certain cases, once the TM5-IC3-TM6 segment is identified for a GPCR,a suitable region of amino acids is chosen for substitution with theamino acid sequence of the a stable, folded protein insertion. Incertain embodiments, the substituted region may be identified usingconserved or semi-conserved amino acids in the TM5 and TM6 transmembraneregions. In certain embodiments, the N-terminus of the a stable, foldedprotein insertion is linked to the amino acid that is 15 to 25 (e.g.,15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25; e.g., 18-20) residuesC-terminal to a conserved proline in the TM5 of the GPCR, althoughlinkages outside of this region are envisioned. In certain embodiments,the C-terminus of the stable, folded protein insertion may be linked tothe amino acid that is 20-30 (e.g., 20, 21, 22, 23, 24, 25, 26, 27, 28,29 or 30; e.g., 23-27) residues N-terminal a conserved proline in theTM6 region of the GPCR, although linkages outside of this region areenvisioned.

For GPCRs that contain no conserved proline residues in TM5 and TM6,positions for inserting an a stable, folded protein insertion can bedetermined based on two considerations: a) alignment of the sequence ofthe GPCR with receptor members of the same subfamily (which containedconserved proline residues in TM5 or TM6; b) by identifying thejuxtaposition to the TM5/TM6 regions by hydrophobicity analysis.

In addition to substituting IC3 region of a GPCR with a stable, foldedprotein insertion, as described above, in certain cases, the C-terminalregion of the GPCR (which is C-terminal to the cysteine palmitoylationsite that is approximately 10 to 25 amino acid residues downstream of aconserved NPXXY motif), may be deleted. In certain cases, the 20-30amino acids immediately C-terminal to the cysteine palmitoylation siteare not deleted.

Stable, Folded Protein Insertions

In certain embodiments, a stable, folded protein insertion of a subjectfusion protein may be a soluble, stable protein (e.g., a proteindisplaying resistance to thermal and chemical denaturation) that foldsautonomously of the GPCR portion of the fusion protein, in a cell. Incertain cases, the stable, folded protein insertion may have no cysteineresidues (or may be engineered to have no cysteine residues) in order toavoid potential disulphide bonds between the stable, folded proteininsertion and a GPCR portion of the fusion protein, or internaldisulphide bonds. Stable, folded protein insertions are conformationallyrestrained, and are resistant to protease cleavage.

In certain cases, stable, folded protein insertions may contain most orall of the amino acid sequence of a polypeptide that is readilycrystallized. Such proteins may be characterized by a large number ofdeposits in the protein data bank (www.rcsb.org) in a variety of spacegroups and crystal packing arrangements. While examples that employlysozyme as stable, folded protein insertion are discussed below, thegeneral principles may be used to employ any of a number of polypeptidesthat have the characteristics discussed above. Suitable stable, foldedprotein insertion candidates include those containing the amino acidsequence of proteins that are readily crystallized including, but notlimited to: lysozyme, glucose isomerase, xylanase, trypsin inhibitor,crambin, ribonuclease. Other suitable polypeptides may be found at theBMCD database (Gilliland et al 1994. The Biological MacromoleculeCrystallization Database, Version 3.0: New Features, Data, and the NASAArchive for Protein Crystal Growth Data. Acta Crystallogr. D50 408-413),as published to the world wide web.

In certain embodiments, the stable, folded protein insertion used may beat least 80% identical (e.g., at least 85% identical, at least 90%identical, at least 95% identical or at least 98% identical to a wildtype protein. Many suitable wild type proteins, including non-naturallyoccurring variants thereof, are readily crystallizable.

As noted above, one such stable, folded protein insertion that may beemployed in a subject fusion protein is lysozyme. Lysozyme is a highlycrystallizable protein (see, e.g., Strynadka et al Lysozyme: a modelenzyme in protein crystallography EXS 1996 75: 185-222) and at presentover 200 atomic coordinates for various lysozymes, including manywild-type lysozymes and variants thereof, including lysozymes from phageT4, human, swan, rainbow trout, guinea fowl, soft-shelled turtle, tapesjaponica, nurse shark, mouse sperm, dog and phage P1, as well asman-made variants thereof, have been deposited in NCBI's structuredatabase. A subject fusion protein may contain any of a wide variety oflysozyme sequences.

The length of the stable, folded protein insertion may be between 80-500amino acids, e.g., 100-200 amino acids in length, although stable,folded protein insertions having lengths outside of this range are alsoenvisioned.

As noted above, the stable, folded protein insertion is not fluorescentor light-emitting. As such, the stable, folded protein insertion is notCFP, GFP, YFP, luciferase, or other light emitting, fluorescent variantsthereof. In certain cases, a stable, folded protein insertion regiondoes not contain a flexible polyglycine linker or other suchconformationally unrestrained regions. In certain cases, the stable,folded protein insertion contains a sequence of amino acids from aprotein that has a crystal structure that has been solved. In certaincases, the stable, folded protein insertion should not have highlyflexible loop region characterized by high cyrstallographic temperaturefactors (i.e., high B-factors).

In general terms, once a suitable polypeptide is identified, a stable,folded protein insertion may be designed by deleting amino acid residuesfrom the N-terminus, the C-terminus or both termini of the polypeptidesuch that the closest alpha carbon atoms in the backbone at the terminiof the polypeptide are spaced by a distance of in the range of 6 Å to 16Å, e.g., 7 Å to 15 Å, 7 Å to 10 Å, 12 Å to 15 Å, 10 Å to 13 Å, or about11 Å (i.e. 10 Å to 12 Å). The stable, folded protein insertion, disposedbetween the TM5 and TM6 regions of a GPCR, spaces those regions by thatdistance. The distance may be modified by adding or removing amino acidsto or from the stable, folded protein insertion.

Amino acid sequence for exemplary lysozyme fusion proteins are set forthin FIG. 9, and the amino acid sequences of exemplary alternativeinsertions (which may be substituted into any of the sequences of FIG. 9in place of the lysozyme sequence) are shown in FIG. 16. These sequencesinclude the sequences of trypsin inhibitor, calbindin, barnase, xylanaseand glucokinase although other sequences can be readily used.

Nucleic Acids

A nucleic acid comprising a nucleotide sequence encoding a subjectfusion protein is also provided. A subject nucleic acid may be producedby any method. Since the genetic code and recombinant techniques formanipulating nucleic acid are known, the design and production ofnucleic acids encoding a subject fusion protein is well within the skillof an artisan. In certain embodiments, standard recombinant DNAtechnology (Ausubel, et al, Short Protocols in Molecular Biology, 3rded., Wiley & Sons, 1995; Sambrook, et al., Molecular Cloning: ALaboratory Manual, Second Edition, (1989) Cold Spring Harbor, N.Y.)methods are used.

For example, site directed mutagenesis and subcloning may be used tointroduce/delete/substitute nucleic acid residues in a polynucleotideencoding GPCR. In other embodiments, PCR may be used. Nucleic acidsencoding a polypeptide of interest may also be made by chemicalsynthesis entirely from oligonucleotides (e.g., Cello et al., Science(2002) 297:1016-8).

In certain embodiments, the codons of the nucleic acids encodingpolypeptides of interest are optimized for expression in cells of aparticular species, particularly a mammalian, e.g., human, species.Vectors comprising a subject nucleic acid are also provided. A vectormay contain a subject nucleic acid, operably linked to a promoter.

A host cell (e.g., a host bacterial, mammalian, insect, plant or yeastcell) comprising a subject nucleic acid is also provided as well aculture of subject cells. The culture of cells may contain growthmedium, as well as a population of the cells. The cells may be employedto make the subject fusion protein in a method that includes culturingthe cells to provide for production of the fusion protein. In manyembodiments, the fusion protein is directed to the plasma membrane ofthe cell, and is folded into its active form by the cell.

The native form of a subject fusion protein may be isolated from asubject cell by conventional technology, e.g., by precipitation,centrifugation, affinity, filtration or any other method known in theart. For example, affinity chromatography (Tilbeurgh et al., (1984) FEBSLett. 16:215); ion-exchange chromatographic methods (Goyal et al.,(1991) Biores. Technol. 36:37; Fliess et al., (1983) Eur. J. Appl.Microbiol. Biotechnol. 17:314; Bhikhabhai et al., (1984) J. Appl.Biochem. 6:336; and Ellouz et al., (1987) Chromatography 396:307),including ion-exchange using materials with high resolution power (Medveet al., (1998) J. Chromatography A 808:153; hydrophobic interactionchromatography (Tomaz and Queiroz, (1999) J. Chromatography A 865:123;two-phase partitioning (Brumbauer, et al., (1999) Bioseparation 7:287);ethanol precipitation; reverse phase HPLC; chromatography on silica oron a cation-exchange resin such as DEAE; chromatofocusing; SDS-PAGE;ammonium sulfate precipitation; or size exclusion chromatography using,e.g., Sephadex G-75, may be employed.

In particular embodiments, the GPCR, e.g., the N- or C-terminus of theGPCR or an external loop of the GPCR, may be tagged with an affinitymoiety, e.g., a his tag, GST, MBP, flag tag, or other antibody bindingsite, in order to facilitate purification of the GPCR fusion protein byaffinity methods.

Before crystallization, a subject fusion protein may be assayed todetermine if the fusion protein is active, e.g., can bind ligand andchange in conformation upon ligand binding, and if the fusion protein isresistant to protease cleavage. Such assays are well known in the art.

In certain cases the subject fusion protein may be combined with aligand for the GPCR of the fusion protein prior to crystallization.

Crystallization Methods

A subject fusion protein may be crystallized using any of a variety ofcrystallization methods, many of which are reviewed in Caffrey Membraneprotein crystallization. J Struct. Biol. 2003 142:108-32. In generalterms, the methods are lipid-based methods that include adding lipid tothe fusion protein prior to crystallization. Such methods havepreviously been used to crystallize other membrane proteins. Many ofthese methods, including the lipidic cubic phase crystallization methodand the bicelle crystallization method, exploit the spontaneousself-assembling properties of lipids and detergent as vesicles(vesicle-fusion method), discoidal micelles (bicelle method), and liquidcrystals or mesophases (in meso or cubic-phase method). Lipidic cubicphases crystallization methods are described in, for example: Landau etal, Lipidic cubic phases: a novel concept for the crystallization ofmembrane proteins. Proc. Natl. Acad. Sci. 1996 93: 14532-5; Gouaux, It'snot just a phase: crystallization and X-ray structure determination ofbacteriorhodopsin in lipidic cubic phases. Structure. 1998 6:5-10;Rummel et al, Lipidic Cubic Phases: New Matrices for theThree-Dimensional Crystallization of Membrane Proteins. J. Struct. Biol.1998 121:82-91; and Nollert et al Lipidic cubic phases as matrices formembrane protein crystallization Methods. 2004 34:348-53, whichpublications are incorporated by reference for disclosure of thosemethods. Bicelle crystallization methods are described in, for example:Faham et al Crystallization of bacteriorhodopsin from bicelleformulations at room temperature. Protein Sci. 2005 14:836-40. 2005 andFaham et al, Bicelle crystallization: a new method for crystallizingmembrane proteins yields a monomeric bacteriorhodopsin structure. J MolBiol. 2002 Feb. 8; 316(1):1-6, which publications are incorporated byreference for disclosure of those methods.

Also provided is a method of determining a crystal structure. Thismethod may comprise receiving an above described fusion protein,crystallizing the fusion protein to produce a crystal; and obtainingatomic coordinates of the fusion protein from the crystal. The fusionprotein may be received from a remote location (e.g., a differentlaboratory in the same building or campus, or from a different campus orcity), and, in certain embodiments, the method may also comprisetransmitting the atomic coordinates, e.g., by mail, e-mail or using theinterne, to the remote location or to a third party.

In other embodiments, the method may comprise forwarding a fusionprotein to a remote location where the protein may be crystallized andanalyzed, and receiving the atomic coordinates of the fusion protein.

In order to further illustrate the present invention, the followingspecific examples are given with the understanding that they are beingoffered to illustrate the present invention and should not be construedin any way as limiting its scope.

Methods

Molecular Biology for Generation of Mammalian and Sf9 ExpressionConstructs.

The insect cell expression plasmid that was used as a template formodification of the human β₂AR gene has been described previously (X.Yao et al., Nat Chem Biol 2, 417 (2006)): the wild-type coding sequenceof the human β₂AR (starting at Gly2) was cloned into the pFastbac1 Sf-9expression vector (Invitrogen) with the HA signal sequence followed bythe Flag epitope tag at the amino terminus and the third glycosylationsite mutated as N187E. Using this template, a TAA stop codon was placedbetween Gly365 and Tyr366, terminating translation without the 48C-terminal residues of the wild-type β₂AR (“βAR365”). A synthetic DNAcassette encoding the T4 Lysozyme (WT*-C54T, C97A) protein was made byoverlapping extension PCR of 50-base oligonucleotides. This cassette wasamplified and inserted into the β₂AR365 construct between Ile233^(5.72)and Arg260^(6.22) (“E1” in FIG. 3A), using the Quickchange Multiprotocol (Stratagene). The corresponding mammalian cell expressionplasmid was made by amplifying the entire fusion gene and cloning itinto pcDNA3 (Invitrogen). Further deletions in the Sf9 and mammaliancell constructs were made using appropriate synthetic oligonucleotidesin the Quickchange Multi protocol (Stratagene). All constructs wereconfirmed by sequencing.

HEK293 Cell Staining and Immunofluorescence Staining.

HEK293 cells were cultured on plastic dishes at 37° C. with 5% CO₂ inDulbecco's modified Eagle's medium (Cellgro) with 5% fetal bovine serum.For an individual expression experiment, cells at confluency were split,and approximately 100,000 cells were used to seed glass cover slips inthe same medium. After 2 d, cells were transfected with the addition of1 μg of a given pcDNA3-receptor plasmid and 3 μl of Fugene 6 reagent(Roche). 48 h after transfection, cells were washed with PBS, fixed with4% paraformaldehyde, blocked with PBS+2% goat serum, permeabilized withPBS+2% goat serum +0.5% Nonidet P-40 (Sigma), stained withAlexa488-conjugated M1 anti-FLAG antibody (for receptor) plus DAPI(nuclear) in blocking buffer, and washed with blocking buffer. Coverslips were mounted on microscope slides with Vectashield (Vector Labs)and dried overnight. Staining was visualized with an Axioplan 2fluorescence imaging system, using a 63× objective and either green(Alexa488/FITC) or blue (DAPI/Hoechst) filter sets. A plasmidpcDNA3-β₁AR, expressing an N-terminal FLAG-tagged β₁ adrenergicreceptor, was used as a positive control for cell-surface staining.Empty pcDNA3 was used as a negative control to assess backgroundstaining.

Expression and Purification of β₂AR-T4L from Baculovirus-Infected Sf9Cells.

Recombinant baculovirus was made from pFastbac1-β₂AR-T4L using theBac-to-Bac system (Invitrogen), as described previously (X. Yao et al.,Nat Chem Biol 2, 417 (2006)). The β₂AR-T4L protein was expressed in Sf9insect cells infected with this baculovirus, and solubilized accordingto previously described methods (B. K. Kobilka, Anal Biochem 231, 269(1995)). Dodecylmaltoside-solubilized receptor with the N-terminal FLAGepitope (DYKDDDA) was purified by M1 antibody affinity chromatography(Sigma), treated with TCEP/iodoacetamide, and further purified byalprenolol-Sepharose chromatography (2) to isolate only functional GPCR.Eluted alprenolol-bound receptor was re-bound to M1 FLAG resin, andligand exchange with 30 μM carazolol was performed on the column.β₂AR-T4L was eluted from this final column with 0.2 mg/ml FLAG peptidein HLS buffer (0.1% dodecylmaltoside, 20 mM Hepes, 100 mM NaCl, pH 7.5)plus 30 μM carazolol and 5 mM EDTA. N-linked glycolsylations wereremoved by treatment with PNGaseF (NEB). Protein was concentrated from˜5 mg/ml to 50 mg/ml with a 100 kDa molecular weight cut-off Vivaspinconcentrator (Vivascience), and dialyzed against HLS buffer plus 10 μMcarazolol.

Binding Measurements on Wild-Type β₂AR and β₂AR-T4L from Membranes.

Membrane preparation from baculovirus-infected Sf9 cells was performedas described previously (G. Swaminath, J. Steenhuis, B. Kobilka, T. W.Lee, Mol Pharmacol 61, 65 (2002)). For each binding reaction, membranescontaining 0.7 μg total membrane protein were used. Saturation bindingof [³H]-dihydroalprenolol (DHA) was measured by incubating membranesresuspended in 500 μl binding buffer (75 mM Tris, 12.5 mM MgCl₂, 1 mMEDTA, pH 7.4, supplemented with 0.4 mg/ml BSA) with 12 differentconcentrations of [³H]DHA (Perkin Elmer) between 20 pM and 10 nM. After1 h incubation with shaking at 230 rpm, membranes were filtered from thebinding reactions with a Brandel harvester, washed with binding buffer,and measured for bound [³H]DHA with a Beckman LS6000 scintillationcounter. Non-specific binding was assessed by performing identicalreactions in the presence of 1 μM alprenolol. For competition binding,membranes resuspended in 500 μA binding buffer were incubated with 0.5nM [³H]DHA plus increasing concentrations of the competing ligand (allcompounds were purchased from Sigma). For (−)-isoproterenol and(−)-epinephrine, concentrations were 100 pM-1 mM, each increasing by afactor of 10. For salbutamol, concentrations were 1 nM-10 mM. ForICI-118,551 and formoterol, concentrations were 1 pM-10 μM. Non-specificbinding was measured by using 1 μM unlabeled alprenolol as competingligand. Each data point in the curves in FIGS. 4A and 10 represents themean of three separate experiments, each done in triplicate. Bindingdata were analyzed by nonlinear regression analysis using GraphpadPrism. The values for K_(d) of [³H]DHA and K_(i) of other ligands areshown in Table S1.

Bimane Fluorescence Experiments on Purified, Detergent-SolubilizedReceptors

β₂AR-T4L and β₂AR365 were purified as described above, with twodifferences. First, prior to iodoacetamide treatment, FLAG-pure receptorat 2.5 μM (measured by soluble [³H]DHA binding) was incubated with 5 μMmonobromobimane for 1 h at 4° C. Second, after binding thebimane-labeled alprenolol-Sepharose-purified receptor to M1 antibodyresin, the column was washed extensively with ligand-free buffer beforeelution. Based on previous precedent, this protocol is expected totarget primarily Cys265^(6.27) for fluorophore derivitization.Fluorescence spectroscopy was performed on a Spex FluoroMax-3spectrofluorometer (Jobin Yvon Inc.) with photon-counting mode, using anexcitation and emission bandpass of 5 nm. All experiments were done at25° C. For emission scans, we set excitation at 350 nm and measuredemission from 417 to 530 nm with an integration time of 1.0 s nm⁻¹. Todetermine the effect of ligands, spectra were measured after 15 minincubation with different compounds (at saturatingconcentrations−[(−)-isoproterenol]=100 μM, [ICI-118,551]=10 μM,[salbutamol]=500 μM). Fluorescence intensity was corrected forbackground fluorescence from buffer and ligands in all experiments. Thecurves shown in FIG. 4B are each the average of triplicate experimentsperformed in parallel. λ_(max) values and intensity changes for β₂AR-T4Land β₂AR365, each incubated with different ligands, are tabulated inTable S2.

Comparing the Proteolytic Stability of Unliganded β₂AR and β₂AR-T4L.

The limited trypsin proteolysis protocol was adapted from Jiang et al.(Z. G. Jiang, M. Carraway, C. J. McKnight, Biochemistry 44, 1163(2005)). Carazolol-bound β₂AR-T4L or wild-type β₂AR (each at 30 mg/ml)were diluted 10-fold into HLS buffer (see above) and TPCK-trypsin wasadded at a 1:1000 ratio (wt:wt). The digests were incubated at roomtemperature. At various time points, aliquots were removed and flashfrozen on dry ice/ethanol. After the last aliquot was removed, allsamples were thawed, and an equal volume of 10% SDS/PAGE loading bufferwas added to each. Samples were then analyzed by electrophoresis on 12%polyacrilamide gels, followed by staining with Coomassie blue. See FIG.11.

Comparing the Stability of Unliganded β₂AR and β₂AR-T4L

Unliganded β₂AR365 and β₂AR-T4L were each purified as described abovefor the bimane experiments. 200 μl 0.02 mg/ml receptor in HLS buffer wasincubated at 37° C. on a heating block. At the time points indicated inFIG. 12, samples were briefly spun and gently vortexed and 16.5 μl wasremoved and diluted 18.2-fold in HLS (300 μl total). Then 4×5 μl wasremoved for determination of total binding and 2×5 μl was removed fornonspecific binding. To measure soluble binding, 5 μl diluted receptorwas added to 105 μl HLS (400-fold final dilution of receptor) containing10 nM [³H]DHA±10 μM cold alprenolol. Reactions were incubated 30 min atRT, then on ice until processing. 100 μl of each reaction was applied toa 1 ml G50 column to separate protein from residual unbound [³H]DHA, andreceptor was eluted using 1.1 ml ice-cold HLS. Bound [³H]DHA wasquantified on a Beckman LS6000 scintillation counter.

Carazolol Dissociation from the “Wild-Type” Receptor β₂AR365

β₂AR365 was purified with carazolol bound, according to the protocoldescribed above for β₂AR-T4L. Carazolol-bound receptor (at approximately50 μM concentration) was dialyzed in the dark against 1 L dialysisbuffer (20 mM HEPES pH7.5, 100 mM NaCl, 0.1% dodecylmaltoside, 300micromolar alprenolol) at room temperature with stirring. At indicatedtime points, two samples were removed from the parafilm-sealedopen-ended dialysis chamber, diluted into fresh dialysis buffer, andcarazolol emission spectra were obtained on a Spex FluoroMaxspectrofluorometer (using excitation at 330 nm and emission from 335 to400 nm). As internal standards for every time point, samples wereremoved for determination of protein concentration using the Bio-RadProtein DC kit. See FIG. 14.

CAM and UCM Mutants

The CAMs (constitutively active mutants) described in the literaturethat are the basis for FIG. 8A and the associated discussion are: L124A,C116F, D130A, L272C, and C285T. The UCMs (uncoupling mutations) from theliterature that were used are: D79N, F139A, T1641, N318K, N322A, P323A,Y326A, L339A, and L340A.

TABLE S1 Binding affinities of different ligands for the wild-type β₂ARand the fusion protein β₂AR-T4L. The saturation and competition bindingcurves shown in FIG. 4 were fit to theoretical saturation and one-sitecompetition binding models, using the program Graphpad Prism. K_(i)values were calculated using the Cheng-Prusoff equation: K_(i) =IC₅₀/(1 + [ligand]/K_(d)) Saturation Binding [³H]DHA K_(d) ± SE (nM)Bmax (pmol/mg) β₂AR 0.161 ± 0.012 30.0 ± 0.5 β₂AR-T4L 0.180 ± 0.016 21.6± 0.5 Competition Binding K_(i) [S.E. interval] for K_(i) [S.E.interval] for Ligand β₂AR (nM) β₂AR-T4L (nM) (−)-isoproteronol  50.6[48.9-52.3]  15.7 [15.2-16.2] (−)-epinephrine   175 [163-188]  56.0[52.8-59.4] salbutamol   728 [708-750]   307 [291-323] ICI-118,551 0.617[0.570-0.668] 0.626 [0.591-0.662] formoterol  3.60 [3.39-3.83]  1.68[1.55-1.81]

TABLE S2 Bimane fluorescence responses for unliganded β₂AR365 andβ₂AR-T4L, incubated for 15 min with different ligands. Top panel showsthe λ_(max) for fluorescence emission spectra (excitation at 350 nm andemission from 417 to 530 nm) collected after 15 min incubation withligand. Each value is mean ± standard deviation for triplicateexperiments performed in parallel. Bottom panel shows the change influorescence intensity after incubation with ligand, represented as theratio of Intensity at λmax of the ligand to Intensity at λ_(max) of thecontrol no ligand (“none”) response. λmax ± SD λmax ± SD for for β₂AR-Ligand β₂AR365 (nm) T4L (nm) none 448 ± 2 447 ± 2 (−)-isoproteronol 453± 2 455 ± 2 ICI-118,551 447 ± 1 446 ± 1 salbutamol 449 ± 1 449 ± 1Intensity at λmax_(Ligand)/Intensity at λmax_(none) Ligand β₂AR365β₂AR-T4L (−)-isoproteronol 0.758 ± 0.007 0.824 ± 0.006 ICI-118,551 1.013± 0.008 1.028 ± 0.008 salbutamol 0.950 ± 0.013 0.928 ± 0.009

TABLE S3 Buried surface area contributions at the β₂AR-T4L/carazololinterface. Solvent accessible surface area calculations were done withthe CNS software package, using a probe radius of 1.4 Å. Buried surfacearea contributions of individual residues were determined by calculatingsolvent-accessible surface area per residue for the full β₂AR-T4L/carazolol model, and subtracting these numbers from the calculatedvalues for the receptor model without carazolol. Surface area β₂ARresidue buried (Å²) Trp109^(3.28) 21.4 Thr110^(3.29) 5.7 Asp113^(3.32)19.3 Val114^(3.33) 25.5 Val117^(3.36) 8.5 Thr118^(3.37) 1.9Phe193^(5.32) 51.2 Thr195^(5.34) 7.4 Tyr199^(5.38) 7.6 Ala200^(5.39)10.0 Ser203^(5.42) 9.0 Ser204^(5.43) 4.6 Ser207^(5.46) 6.3 Trp286^(6.48)3.1 Phe289^(6.51) 20.0 Phe290^(6.52) 19.0 Phe293^(6.55) 18.7Tyr308^(7.35) 14.4 Asn312^(7.39) 22.5 Tyr316^(7.43) 6.5

Lipidic Cubic Phase Crystallization

For lipidic cubic phase (LCP) crystallization trials, robotic trialswere performed using an in meso crystallization robot. 96-well glasssandwich plates (S1, S2) were filled with 25 or 50 nL protein-laden LCPdrops overlaid by 0.8 μL of precipitant solution in each well and sealedwith a glass coverslip. All operations starting from mixing lipid andprotein were performed at room temperature (˜21-23° C.). Crystals wereobtained in 30-35% (v/v) PEG 400, 0.1-0.2 M sodium sulfate, 0.1 MBis-tris propane pH 6.5-7.0 and 5-7% (v/v) 1,4-butanediol using 8-10%(w/w) cholesterol in monoolein as the host lipid. PEG 400 and sulfateion were used for crystallization, and the addition of cholesterol and1,4-butanediol improved crystals size and shape enabling high-resolutiondiffraction. Additions of phospholipids (dioleoylphosphatidylcholine,dioleoylphosphatidylethanolamine, asolectin) alone and in combinationswith cholesterol to the main host LCP lipid monoolein were tried,however, none of them improved crystal quality.

Crystal Harvesting

The average size of the harvested crystals was 30×15×5 μm (largestcrystal was 40×20×7 μm). Crystals were harvested directly from the glasssandwich plates, even though these plates have been specificallydesigned for screening and optimization (S1, S2). Crystals were scoopeddirectly from the LCP using 30 or 50 μm aperture MiTeGen MicroMounts andplunged into liquid nitrogen. Care was taken to drag as little aspossible lipid around the crystal to decrease unwanted backgroundscattering. Attempts to dissolve the lipids, either by increasingconcentration of PEG 400 or using a mineral oil, typically resulted in adecrease in diffraction power of the crystals.

Data Collection

X-ray data were collected on the 231D-B beamline (GM/CA CAT) at theAdvanced Photon Source, Argonne, IL using a 10 μm minibeam (wavelength1.0332 Å) and a MarMosaic 300 CCD detector. Several complete datasetswere collected from single crystals at resolution between 2.8 and 3.5 Åusing 5× attenuated beam, 5 s exposure and 1° oscillation per frame.However, some crystals diffracted to a maximum of 2.2 Å resolution upon5 s exposure with 1× attenuated beam. Therefore, we collected 10-20°wedges of high-resolution data from more than 40 crystals (some of thecrystals were large enough to allow 2-3 translations) and combined 31 ofthe best datasets together from 27 independent crystals, scaling themagainst the lower resolution full dataset to obtain complete 2.4 Å data.

One of the challenges during data collection was visualization ofcolorless microcrystals within an opaque frozen lipid phase and aligningthem with the 10 μm minibeam. Without being able to visualize thecrystals adequately through the inline optics at the beamline, weresorted to alignment by diffraction. After numerous trial-and-errorattempts, an optimized crystal search algorithm was designed to locatethe crystals without the minibeam. First, the area of the loopcontaining lipid was scanned in the vertical direction with a highlyattenuated and slitted 100×25 μm beam. When diffraction was found, thecrystal location was further confined by two additional exposures to anarea of ˜50×25 μm. This area was further coarse-scanned with thecollimated and 10× attenuated minibeam using 15 μm steps, following byfine-tuning the position using 5 and 2 μm steps. After locating thecrystal in one orientation the loop was rotated 90° and the procedurewas repeated. Typically during alignment the crystal was exposed ˜10times using 10× attenuated beam and 2 s exposures. Work is in progressto develop a fully automated scanning procedure to align invisiblemicrocrystals with the minibeam in place.

Data Processing

A 90% complete, 2-fold redundant monoclinic dataset was processed fromone crystal diffracting to 2.8 Å resolution. Initial indexing of latticeparameters in spacegroup C2 and crystal orientation were performed usingHKL2000. The refined lattice parameters and space group were implementedin the data processing program XDS for spot integration which modelserror explicitly for radiation decay, absorption, and rotation. The 2.8Å data was used as a scaling reference for incorporation of additionalwedges of data collected at a much higher exposure. Each new dataset wasindexed in XDS using the original unit cell parameters as constantswhich were then refined along with the crystal orientation, beamgeometry, and mosaicity parameters. The refinement was generally stable,resulting in very similar unit cell constants which enabled subsequentscaling. All of the integrated wedges of data were then testedindividually against the scaling reference set and included in the finalscaled dataset if the merging statistics remained acceptable uponincorporation of the data. In total, 31 wedges of data from 27 crystalswere combined with the scaling reference dataset, 22 of which diffractedto a resolution of 2.4 Å or better. Each of the higher resolutiondatasets were exposed to a much larger dose of radiation resulting in arapid decay in intensity. Typically 10°-20° wedges were collected fromeach crystal or translation, 5°-7° of which had diffraction data to 2.4Å. Based on the mean F/σ(F) of reflections near the threecrystallographic axes, we estimate the effective resolution to be 2.4 Åalong b* and c* and 2.7 Å along a*. The anisotropy results in the highmerging R factors in the last few resolution shells despite thesignificant I/σ(I) values. The anisotropy is either an inherent propertyof the crystals or the result of a preferential orientation of thecrystals within the mounting loop. Thus, the higher resolution shellswere filled in anisotropically by incorporation of the additional dataat high exposure levels, while the lower resolution shells have a veryhigh redundancy and low anisotropy.

Example 1 Summary of Results

In order to obtain high-resolution structural information on the β₂AR,most of the third intracellular loop (ICL3) was replaced by the proteinT4 lysozyme (T4L). The C-terminal tail was also eliminated. Theoptimized β₂AR-T4L protein was crystallized in lipidic cubic phase, andthe resulting 2.4 Å resolution crystal structure reveals the interfacebetween the receptor and the ligand carazolol, a partial inverseagonist. Analysis of mutagenesis data in light of the structureclarifies the roles of different amino acids in inverse agonist binding,and implies that rearrangement of the binding pocket accompanies agonistbinding. In addition, the structure reveals how mutations known to causeconstitutive activity or uncoupling of agonist binding and G-proteinactivation are distributed between the ligand-binding pocket and thecytoplasmic surface of the protein, such that changes in side chains dueto interaction with the ligand can be transmitted through the structureto the site of G protein interaction.

Example 2 β₂AR-T4L A Crystallizable GPCR Fusion Protein

β₂AR crystallization was done by replacing the ICL3 of that protein witha well-structured, soluble domain that aids in the formation of latticecontacts. The initial criteria for choosing the inserted soluble proteinwere that the amino and carboxyl termini would approximate the predicteddistance between the cytoplasmic ends of helix V and helix VI, and thatthe protein would crystallize under a variety of conditions. T4L is asmall, stable protein that fulfills these criteria. The amino andcarboxyl termini of wild-type T4L are 10.7 Å apart in PDB 2LZM, comparedto a distance of 15.9 Å between the carbonyl carbon of residue228^(5.63) and the amide nitrogen of residue 241^(6.24) in thehigh-resolution structure of rhodopsin (PDB 1U19).

DNA encoding the T4L protein (C54T, C97A) (M. Matsumura, W. J. Becktel,M. Levitt, B. W. Matthews, Proc Natl Acad Sci USA 86, 6562 (1989)) wasinitially cloned into the human β₂AR gene, guided by comparison of ICL3length and sequence among class A GPCRs (F. Horn et al., Nucleic AcidsRes 31, 294 (2003)): residues 234^(5.73)-259^(6.21) of the β₂AR werereplaced by residues 2-164 of T4L (construct “E3” in FIG. 3A). Inaddition, the receptor was truncated at position 365, which alignsapproximately with the position of the rhodopsin carboxyl terminus.Although these modifications resulted in a receptor that was expressedefficiently in Sf9 cells, further optimization was carried out to reducethe length of the junction between the receptor and the T4L termini.Several candidate constructs are illustrated in FIG. 3A, and selectedimmunofluorescence images of transfected, permeabilized HEK293 cells areshown in FIG. 3B. Relative to the initial construct, we could removethree residues from the cytoplasmic end of helix V, three residues fromthe C-terminal end of T4L, and three residues from the N terminus ofhelix VI, all without losing significant cell-surface expression. Thefinal construct used for crystallization trials (“β₂AR T4L”) hasresidues 231^(5.70)-262^(6.24) of the β₂AR replaced by amino acids 2-161of T4L (“1D” in FIG. 3A).

Example 3 Functional Properties of β₂AR-T4L

Saturation binding of [³H]DHA to the β₂AR-T4L was measured, as well ascompetition binding of the inverse agonist ICI-118,551 and severalagonists (FIGS. 4A and 10 and Table S1). The results show that β₂AR-T4Lhas wild-type affinity for the antagonist [³H]DHA and the inverseagonist ICI-118,551, whereas the affinity for both agonists(isoproterenol, epinephrine, formoterol) and a partial agonist(salbutamol) is two to three-fold higher relative to wild-type β₂AR.Higher agonist binding affinity is a property associated withconstitutively active mutants (CAMs) of GPCRs. CAMs of the β₂AR alsoexhibit elevated basal, agonist-independent activation of Gs, andtypically have lower expression levels and reduced stability. β₂AR-T4Lexhibits binding properties of a CAM, but it expresses at levelsexceeding 1 mg per liter of Sf9 cell culture, is more resistant totrypsin proteolysis than the wild-type β₂AR (FIG. 11), and retainsbinding activity in detergent at 37° C. as well as the wild-typereceptor (FIG. 12).

β₂AR-T4L did not couple to G_(s), as expected due to the replacement ofICL3 by T4L. To assess whether the fused protein alters receptorfunction at the level of its ability to undergo conformational changes,we used a covalently attached fluorescent probe as a reporter forligand-induced structural changes. Fluorophores attached atCys265^(6.27), at the cytoplasmic end of helix VI, detectagonist-induced conformational changes that correlate with the efficacyof the agonist towards G protein activation. Detergent-solubilizedβ₂AR365 (wild-type receptor truncated at 365) and β₂AR-T4L were eachlabeled with monobromobimane, which has been used previously to monitorconformational changes of the β₂AR. Addition of the agonistisoproterenol to purified β₂AR365 induces a decrease in fluorescenceintensity and a shift in λ_(max) for the attached bimane probe (FIG. 4Band Table S2). These changes in intensity and λ_(max) are consistentwith an agonist-induced increase in polarity around bimane. A smallerchange is observed with the partial agonist salbutamol, while theinverse agonist ICI-118,551 had little effect. For the β₂AR-T4L, thereare subtle differences in the baseline spectrum of the bimane-labeledfusion protein, as might be expected if the environment aroundCys265^(6.27) is altered by T4L. However, the full agonist isoproterenolinduces a qualitatively similar decrease in intensity and rightwardshift in λ_(max). Thus the presence of the fused T4L does not preventagonist-induced conformational changes. The partial agonist salbutamolinduced larger responses in β₂AR-T4L than were observed in wild-typeβ₂AR, and there was a small increase in fluorescence in response to theinverse agonist ICI-118,551. These are properties observed in CAMs andare consistent with the higher affinities for agonists and partialagonists exhibited by β₂AR-T4L. Therefore, we conclude that the T4Lfusion induces a partial constitutively active phenotype in the β₂AR,likely caused by changes at the cytoplasmic ends of helices V and VI.

Example 4 Comparison Between β₂AR-T4L and β₂AR-Fab Structures

The β₂AR-T4L fusion strategy is validated by comparison of its structureto the structure of wild-type β₂AR complexed with a Fab that recognizesa three dimensional epitope consisting of the amino andcarboxyl-terminal ends of ICL3, determined at an anisotropic resolutionof 3.4 Å/3.7 Å. FIG. 5A illustrates the similarity between the fusionand antibody complex approaches to β₂AR crystallization, in that bothstrategies rely on attachment (covalent or non-covalent, respectively)of a soluble protein partner between helices V and VI. A majordifference between the two structures is that the extracellular loopsand the carazolol ligand could not be modeled in the β₂AR-Fab complex,whereas these regions are resolved in the structure of β₂AR-T4L.Nonetheless, it is clear that the T4L insertion does not significantlyalter the receptor. Superposition of the two structures (FIG. 13)illustrates that the transmembrane helices of the receptor componentsare very similar (RMSD=0.8 Å for 154 common modeled transmembrane Cαpositions, versus 2.3 Å between β₂AR-T4L and the 154 equivalent residuesin rhodopsin), especially when the modest resolution of the Fab complexis taken into account.

There is one significant difference between the Fab-complex and chimericreceptor structures that can be attributed to the presence of T4L. Thecytoplasmic end of helix. VI is pulled outward as a result of the fusionto the carboxyl terminus of T4L, which alters the packing ofPhe264^(6.26) at the end of helix VI (FIG. 5B). In the Fab-complex β₂AR,interactions between Phe264^(6.26) and residues in helix V, helix VI,and ICL2 may be important in maintaining the β₂AR in the basal state.The loss of these packing interactions in β₂AR-T4L could contribute tothe higher agonist binding affinity characteristic of a CAM.

An unexpected difference between the structure of rhodopsin and theβ₂AR-T4L involves the sequence E/DRY found at the cytoplasmic end ofhelix III in 71% of class A GPCRs. In rhodopsin, Glu134^(3.49) andArg135^(3.50) form a network of hydrogen bond and ionic interactionswith Glu247^(6.30) at the cytoplasmic end of helix VI. Theseinteractions have been referred to as an “ionic lock” that stabilizesthe inactive state of rhodopsin and other class A members. However, thearrangement of the homologous residues is significantly different inβ₂AR-T4L: Arg131^(3.50) interacts primarily with Asp130^(3.49) and asulfate ion rather than with Glu268^(6.30), and the distance betweenhelix III and helix VI is greater than in rhodopsin (FIG. 5C). Thisdifference might be explained by the interaction between Glu268^(6.30)and Arg8 of T4L; however, the arrangement of Asp130^(3.49) andArg131^(3.50) and the distance between helix III and helix VI is verysimilar to that observed in the β₂AR-Fab structure. While the presenceof an antibody or T4L at the ICL3 region could potentially affect thearrangement of these residues, the fact that similar ionic lockstructures were obtained using two different approaches suggests that abroken ionic lock may be a genuine feature of the carazolol-bound stateof the receptor.

Example 5 Ligand Binding to the β₂AR

The β₂AR-T4L fusion protein was purified and crystallized in complexwith the inverse agonist carazolol. Carazolol stabilizes the β₂ARagainst extremes of pH and temperature, perhaps related to its unusuallyhigh binding affinity (K_(d)<0.1 nM) and slow dissociation kinetics(t_(1/2)˜30 h) (FIG. 14). The interactions between carazolol andβ₂AR-T4L are depicted schematically in FIG. 6. The carbazole ring systemis oriented roughly perpendicular to the plane of the membrane, and thealkylamine chain (atoms 15-22 in the model) is nearly parallel to theheterocycle (FIG. 7A-B). Carazolol was modeled into the electron density(3) as the (S)-(−) isomer due to the higher affinity of this enantiomer,despite the fact that a racemic mixture of the ligand was used incrystallization. Asp113^(3.32), Tyr316^(7.43), and Asn312^(7.39) presenta constellation of polar functional groups to the alkylamine and alcoholmoieties of the ligand, with Asp113^(3.32) and Asn312^(7.39) sidechainsforming close contacts (<3 Å) with O₁₇ and N₁₉ atoms of carazolol (FIGS.6 and 7A-B). Asp113^(3.32) was one of the first β₂AR residues shown tobe important for ligand binding; notably the D113N mutation causescomplete loss of detectable affinity for antagonists and a decrease inthe potency of agonists towards cell-based G protein activation by over4 orders of magnitude. Likewise, mutations of Asn312^(7.39) perturb β₂ARbinding to agonists and antagonists: changes to nonpolar amino acids(Ala or Phe) reduce affinities to undetectable levels, while retentionof a polar functionality (Thr or Gln) gives partial affinity. On theopposite end of the ligand near helix V, N₇ of the carbazole heterocycleforms a hydrogen bond with the side chain hydroxyl of Ser203^(5.42).Interestingly, mutations of Ser203^(5.42) specifically decrease β₂ARaffinity towards catecholamine agonists and aryloxyalkylamine ligandswith nitrogen-containing heterocycles such as pindolol, and byimplication carazolol. Thus, the polar interactions between carazololand the receptor observed in the crystal structure agree with the knownbiochemical data. The contribution of Tyr316^(7.43) to antagonist andagonist affinity remains to be tested; this residue is conserved astyrosine in all sequenced adrenergic receptor genes.

FIG. 7C shows the tight packing between carazolol and surrounding aminoacids that buries 790 Å² of surface area from solvent; specific contactsare depicted schematically in FIG. 6. Notable among the hydrophobicresidues contacting carazolol are Val114^(3.33), Phe290^(6.52), andPhe193^(5.32). The side chain of Val114^(3.33) from helix III makesmultiple contacts with the C₈-C₁₃ ring of the carbazole heterocycle, andPhe290^(6.52) from helix VI forms an edge-to-face aromatic interactionwith the same ring. As a result, these two amino acids form ahydrophobic “sandwich” with the portion of the aryl moiety that iscommon to many adrenergic antagonists. Mutation of Val114^(3.33) toalanine was shown to decrease β₂AR affinity towards the antagonistalprenolol by an order of magnitude, as well as lowering affinity forthe agonist epinephrine 300-fold. Phe193^(5.32) is different from othercarazolol contact residues in that it is located on the ECL2, in thepath of hormone accessibility to the binding pocket. This amino acidcontributes more buried surface area than any other residue to theinterface between β₂AR-T4L and carazolol (see Table S3). Therefore,Phe193^(5.32) is likely to contribute significantly to the energy ofβ₂AR-carazolol complex formation, and the position of this residue onthe extracellular side of the binding site may allow it to act as a gatethat contributes to the unusually slow dissociation of the ligand (FIG.14).

Analysis of the binding pocket provides insights into the structuralbasis for pharmacologic selectivity between the β₂AR and closely relatedadrenergic receptors such as the β₁AR. The affinities of these tworeceptors for certain ligands, such as ICI-118,551, betaxolol and RO363,differ by up to 100-fold. Curiously, all of the amino acids in thecarazolol binding pocket are conserved between the β₁AR and β₂AR (seeFIG. 15). The majority of the 94 amino acid differences between the β₁ARand β₂AR are found in the cytoplasmic and extracellular loops. Whileresidues that differ in the transmembrane segments generally face thelipid bilayer, eight residues lie at the interface between helices andmay influence helix packing. The structural basis for pharmacologicdifferences between β₁AR and β₂AR must, therefore, arise from amino aciddifferences in the entrance to the binding pocket or subtle differencesin the packing of helices. Evidence for the latter comes from chimericreceptor studies in which successive exchange of helices between β₁ARand β₂ARs led to a gradual change in affinity for the β₂AR selectiveICI-118,551 and the β₁AR selective betaxolol.

As discussed above, β₂AR-T4L shows CAM-like properties with respect toagonist binding affinities, suggesting that the unliganded β₂AR-T4L mayexist in a more active conformation than the wild type-β₂AR.Nevertheless, as shown in FIG. 4B, β₂AR-T4L can be stabilized in aninactive conformation by an inverse agonist. Since β₂AR-T4L wascrystallized with bound carazolol, a partial inverse agonist, thestructure most likely represents an inactive state. This is consistentwith the similarity of the β₂AR-T4L and β₂AR-Fab5 carazolol-boundstructures. To assess whether conformational changes are required toaccommodate catecholamines, a model of isoproterenol was placed in thebinding site such that common atoms (16-22 in FIG. 6) were superimposedonto the analogous carazolol coordinates in the crystal structure (FIG.7D). Residues Ser204^(5.43) and Ser207^(5.46) are critical forcatecholamine binding and activation of the β₂AR, with Ser204^(5.43)hydrogen bonding to the meta-hydroxyl and Ser207^(5.46) to thepara-hydroxyl of the catechol ring, respectively. In our model, thecatechol hydroxyls of isoproterenol face the appropriate serines onhelix V, but the distances are too long for hydrogen bonding (6.8 Å frommeta-hydroxyl oxygen to the sidechain oxygen of Ser204^(5.43), 4.8 Åfrom the para-hydroxyl oxygen to the sidechain oxygen of Ser207^(5.46)).In addition, Asn293^(6.55) and Tyr308^(7.35), two residues expected toform selective interactions with agonists based on the literature, aretoo distant to form productive polar or hydrophobic contacts with themodeled isoproteronol molecule. These observations suggest that agonistbinding requires changes in the binding site relative to thecarazolol-bound structure, unless common structural components ofagonists and inverse agonists bind in a significantly different manner.

Example 6 Structural Insights into β₂AR Activation

Biophysical studies provide evidence that conformational changesassociated with activation of the β2AR are similar to those observed forrhodopsin. Yet the highly efficient process of light activation ofrhodopsin through the cis-trans isomerization of covalently boundretinal is very different from activation of the β₂AR and other GPCRs bydiffusible hormones and neurotransmitters. Despite representing a staticpicture of the inverse agonist-bound state, the crystal structure ofβ₂AR-T4L still shows how agonist binding is translated into structuralchanges in the cytoplasmic domains of receptor. Agonist binding occursat the extracellular ends of helices III, IV, V and VII, and G proteinactivation is mediated by the cytoplasmic ends. While the structure isopen at the extracellular face to form the ligand binding pocket, thehelices are more closely packed in the intracellular half of thereceptor. This close packing implies that isolated rigid-body movementof any of these helices is unlikely, and that conformational changes canonly be accomplished by rearrangement of side chains forming the networkof interactions between the helices. Biophysical studies show thatstructurally different agonists stabilize distinct active states,suggesting that different ligands could stabilize different combinationsof side chain rearrangements.

Analysis of mutations that affect β₂AR function provides insights intostructural rearrangements that are likely to occur during receptoractivation. FIG. 8A illustrates the location of amino acids for whichmutations lead to elevated basal, agonist-independent activity(constitutively active mutations, CAMs), as well as amino acids forwhich mutations impair agonist activation (uncoupling mutations, UCMs).Residues for which CAMs have been described are likely to be involved ininteractions that maintain the receptor in the inactive conformation.These amino acids are centrally located on helices III and VI. Incontrast, positions in which UCMs have been observed are likely to formintramolecular interactions that stabilize the active state. A clusterof UCMs are found at the cytoplasmic end of helix VII. Neither CAMs norUCMs are directly involved in agonist binding. Although the CAMs andUCMs are not directly connected in sequence, it is evident from thestructure that they are linked through packing interactions, such thatmovements in one will likely affect the packing of others. For example,FIG. 8A (right panel) shows all amino acids with atoms within 4 Å of thetwo centrally located CAMs, Leu124^(3.43) and Leu272^(6.34). Severalamino acids that pack against these CAMs also interact with one or moreUCMs. Trp286^(6.48) lies at the base of the binding pocket. It has beenproposed that agonist binding leads to a change in the rotameric stateof Trp286^(6.48) with subsequent changes in the angle of the helicalkink formed by Pro288^(6.50). It is likely that an agonist-inducedchange in the rotameric state of Trp286^(6.48) will be linked to changesin sidechains of CAMs and UCMs through packing interactions andpropagated to the cytoplasmic ends of the helices and the associatedintracellular loops that interact with G proteins and other signalingmolecules.

In the structures of both rhodopsin and the β₂AR, a cluster of watermolecules lies near the most highly conserved class A GPCR residues(FIG. 8B). It has been proposed that these water molecules may play arole in the structural changes involved in receptor activation. FIG. 8Cshows the network of potential hydrogen bonding interactions that linkTrp286^(6.48) with conserved amino acids extending to the cytoplasmicends of helices. UCMs have been identified for three amino acids linkedby this network—N322^(7.49), P323^(7.50), and Y326^(7.53). Thisrelatively loose-packed, water filled region is likely to be importantin allowing conformational transitions, as there will be fewer stericrestraints to sidechain repacking. Future structures of theagonist-bound state of the β₂AR will help to clarify the preciserearrangements that accompany activation of the receptor.

1. An isolated GPCR fusion protein comprising, from N-terminus toC-terminus: i. a first portion of a G-protein coupled receptor (GPCR),wherein said first portion comprises TM1, TM2, TM3, TM4 and TM5 regionsof said GPCR; ii. a domain comprising the amino acid sequence of alysozyme; and iii. a second portion of said GPCR, wherein said secondportion comprises TM6 and TM7 regions of said GPCR.
 2. The isolated GPCRfusion protein of claim 1, wherein said isolated GPCR fusion protein ispresent in an isolated membrane.
 3. The isolated GPCR fusion protein ofclaim 1, wherein said isolated GPCR fusion protein is present on thesurface of a cell.
 4. The isolated GPCR fusion protein of claim 1,wherein said isolated GPCR fusion protein is epitope tagged.
 5. Theisolated GPCR fusion protein of claim 1, wherein said isolated GPCRfusion protein is active.
 6. The isolated GPCR fusion protein of claim1, wherein said domain comprises an amino acid sequence having at least80% identity to the amino acid sequence of a wild-type lysozyme.
 7. Theisolated GPCR fusion protein of claim 1, wherein said domain comprisesan amino acid sequence having at least 95% identity to a wild-typelysozyme.
 8. The isolated GPCR fusion protein of claim 1, wherein saiddomain comprises an amino acid sequence that is at least 95% identicalto the amino acid sequence of T4 lysozyme.
 9. The isolated GPCR fusionprotein of claim 1, wherein said first and second portions of said GPCRcomprise the amino acid sequence of a naturally occurring GPCR.
 10. Theisolated GPCR fusion protein of claim 1, wherein said first and secondportions of said GPCR comprise the amino acid sequence of anon-naturally occurring GPCR.
 11. The isolated GPCR fusion protein ofclaim 1, wherein the amino acid sequence of said first and secondportions of said GPCR are least 80% identical to amino acid sequences ofa wild type GPCR.
 12. The isolated GPCR fusion protein acid of claim 1,wherein said a GPCR is selected from the group consisting of: a receptorfor a biogenic amine, a dopamine receptor, a serotonin receptor, anadrenergic receptor, a β2-adrenergic receptor, a melanocortin receptorsubtype 4, a ghrelin receptor, a metabotropic glutamate receptor and achemokine receptor.
 13. The isolated GPCR fusion protein of claim 1,wherein said domain is in the range of from 100 to 200 amino acids inlength.
 14. A polypeptide comprising, from N-terminus to C-terminus: a)a first portion of a G-protein coupled receptor (GPCR), wherein saidfirst portion comprises the amino acid sequence that is N-terminal tothe IC3 loop of said GPCR; b) a domain comprising the amino acidsequence of a lysozyme; c) a second portion of said GPCR, wherein saidsecond portion comprises the amino acid sequence that is C-terminal tothe IC3 loop of said GPCR.
 15. The polypeptide of claim 14, wherein saiddomain spaces the C-terminal end of the first portion of said GPCR andthe N-terminal end of the second region of said GPCR so that the closestalpha carbon atoms at said C-terminal end and said N-terminal end arespaced by a distance of in the range of 6 Å to 16 Å.
 16. An isolatedG-protein coupled receptor (GPCR) comprising an IC3 loop containing asubstitution that comprises the amino acid sequence of a lysozyme.